In addition to your mobo & CPU combination and graphics card power requirements (which are the most variable), you need to add the following:
HDD = 25-35 watts each, depending on speed (5400 / 7200).
DVD RW = 25 Watts each,
Each case fan = 2-3 watts, depending on diam - note that a CPU fan counts!
PCI sound card = 7 watts
Each PCI card = 5 watts (don't count embedded NICs, WIFI, Sound).
LCD panel / permanent LED lights = 2 Watts.
For example, the PC I recently built has an AMD core2 Athlon, ASUS AM2 mobo, 2 x 1GB DDR2 RAM, GeForce 7300, 2 x 7200 HDD, 2 x DVD RW, FDD, PCI wifi NIC, PCI DAB Radio card.
Running my system through 2 calculators gives recommendations of 290 and 340 watt PSU. So the minimum would be 300, probably a 320 would be OK and a 350 would be best.
As it happens I installed a 500W PSU on the basis that I have plenty of headroom to upgrade.
I have several old Compaq PIII 6/7/800 MHz PCs that have proprietary 200W PSUs that are non-upgradeable, which does rather limit the extent to which I can improve them.
For example, I would like to install 2 x HDD, 2 x CD/DVD drives and a 128MB AGP4X vga card into the 866 MHz PC, but the 200W PSU simply isn't up to it.
SD